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  3. Almost everybody wants to live as long as possible. And given the enormous strides made in medicine and the health sciences duri

Almost everybody wants to live as long as possible. And given the enormous strides made in medicine and the health sciences duri

admin2010-06-30  110
问题    Almost everybody wants to live as long as possible. And given the enormous strides made in medicine and the health sciences during the past 150 years, people could be forgiven for hoping that someday human beings will live, if not quite forever, at least far longer than at present. Since the mid 19th century, average life expectancy at birth has nearly doubled: from 40 years to 75. Today many people live past 100, and the oldest individuals have reached either 115 or 120, depending on whom you believe.
   So it comes as something of a surprise to be told by the experts that human beings have taken life about as far as it can go. That is the serious conclusion of a report in Science magazine last week by S. Jay Olshansky and Christine Cassel. Unless an unexpected breakthrough in basic science that would prevent the aging process, the era of rapid increases in human life span has come to an end, at least in developed countries. Even if science could cure heart disease and cancer, which account for nearly 50 percent of all deaths in the U. S. , it is unlikely that the average life expectancy at birth would increase much beyond 85.
   What makes the report so compelling is that it is based on simple mathematics. In the past, the upper limits of life have been guessed from actuarial tables by estimating how death rates would change if, say, the incidence of heart disease were halved. "We reversed the question," says Olshan sky. Taking an" engineering approach" his team members asked themselves how many death rates would have to be reduced in order to increase average life expectancy to 120 years. What they discovered, after running the numbers through a computer, was that big hits in current death rates in the U. S. would give only small lifts to life expectancy. For example, if some miracle of medicine can guarantee no one died before reaching age 50 (thus eliminating 12 percent of all deaths), the increase in average life expectancy would be only 3.5 years.
   There seems to be a kind of built-in biological limit programmed into the cells of the human body. In laboratory experiments, human cells divide only about 50 times before they begin to fall apart like old cars. This planned loss of use on nature’s part makes a certain amount of evolutionary sense. Survival of the fittest, after all, rewards only those who reproduce, not necessarily those who reach old age, Once reproduction is over, human bodies may as well be throwaway goods, biologically speaking.
What is important for human bodies to renew?
选项 A、Evolution.
B、Biological condition.
C、Reproduction.
D、Old age.
答案C
解析 根据文章第的最后一段可知人体的细胞有一个固有的生物限制，试验表明当细胞像旧汽车的零件一样报废之前，细胞要分裂50次。自然生物中这种既定的使用损耗体现了进化的意义。适者生存就是有利于那些能再生的，而不一定是那些最老的。一旦再生停止了，从生物角度讲人体也是可以丢弃的物品了。A、B和D都不是对人体更新重要的因素。
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